Mines: The Ticking Time Bomb under Southwestern Pennsylvania Roads

The title reflects a serious problem that affects the southwest region of Pennsylvania. Coal has been mined in the area for approximately 200 years. As a result, much of the area is undermined by abandoned coal mines and is currently being mined using both room and pillar and longwall techniques.

Room and pillar mining was the most common technique used until about 1980. The mines used wooden posts to support the roof. It is these wooden posts along with the pillars of coal that are decaying and collapsing causing the subsidence of the mine roof. This subsidence is very unpredictable because of the many variables on which it depends. Also, the companies that did the mining for the most part have been dissolved and there is no one to reimburse the owners of the surface facilities for their damages. Public agencies are then called upon to absorb the costs of subsidence remediation.

Since 1980, longwall mining has become the most common mining technique. Longwall mining involves driving parallel sets of chain pillars and then removing all of the coal between the chain pillars. These longwall panels can be as large as 1100 feet wide and several thousand feet long. This mining causes the settlement over the panel to begin immediately after mining. This is a great advantage because it is known when and approximately how much settlement will occur. The owners of surface facilities can prepare for the subsidence and the mining companies responsible for the damage can be required to then repair the damage. The only uncertainty with longwall mining is how the chain pillars will behave over time.

This presentation will give examples of problems that PENNDOT District 12-0 has had with abandoned underground mines and active longwall mines. Monitoring of active longwall mines will also be discussed.